[hahns]

I'm fairly new to CAD/CAM and am having an issue with my latest project.
It's a large horn, too big for my Z axis. To accommodate this, I sliced the model into 3/4" slices and added index holes. The idea was to mill out individual layers and then stack them up, aligned by the index holes. However, things aren't lining up.
Not sure if this is the issue, but I was seeing an error, in Mach 3, "Radius to end of arc differs from radius to start". A quick Google search suggested setting the defaults of INJ in mach3 between absolute and Incremental. Setting this as default in Mach does resolve the error, but I imagine it is the reason for my non-aligning parts.

I am using Fusion 360 with the post output for Mach 3.

The first line of the code that is generated is:
G90 G94 G91.1 G40 G49 G17

I'm super confused by this, as I thought G90=Absolute and G91.1=Incremental? Why would they be on the same line?

Does this sound like it could be the reason I am seeing my parts not align?

[Superman]

G90 = absolute mode
G91 = incremental mode
G91.1 = sets the arc IJK incremental distace back to default behaviour
G91.1 is not the reason for misalignment.

Did you use different sized holes to assist with correct orientation ?
How do you define top face of cut part ?

Your slice, is it a shadow outline ?
-ie looking from top, your outline does not allow any undercuts

[hahns]

Ahhh, that makes sense. Thanks for the info. I didn't realize it was resetting to the default. So, if it is setting it to the default, and I have to change it in Mach 3 in order to get it to cut, wouldn't that effect the cut? I suppose it would be way off if it was referencing the wrong starting coordinates though.

As for the second part of your post, the holes are the same size all the way through. I made them and sliced the model afterwards. I'm not sure what the shadow line is, but I don't believe that is the issue. Here is what the model looks like:

[Superman]

What I was getting too, is that g-codes are grouped, where 1 g-code from a group does NOT affect one in a different group
Ie G90 or G91 is one group
G1 G2 G3 G0 is another

G91.1 is a code to set the IJKs for arcs back to an incremental distance from the start of the arc
-Really, this code is only needed if you are an advanced user, and if you, at times, play with machine parameters.
That 1st line is a safety line, to cover your a$$ against silly errors.
Me, I'd replace G91.1 with G80 (cancel canned cycles) but that's only me.
Your mis-alignment or end point errors are caused by something else. It could be a Fusion setting or a machine setting.